When the heater on my 2004 Chrysler 300M turns on the inside windows fog up. Once the car is warmed up and the heater is blowing hot air the defoggers will work to eliminate the fogging. It just started doing this this year. I have taken it to one of those franchise service shops and they said they don’t see any leaks. I also took it to the Chrysler dealer. All they did was clean the heater box and add .5 freeon. That didn’t work. It doesn’t seem to do it when the temps are warmer. Any suggestions?
There are probably only two scenarios:
You have a leaking heater core
or
You and/or your passengers are not knocking excess snow & ice off of their boots before entering the car
I don’t know how this particular car is wired up, but a third possibility might be that the A/C compressor is not cycling on in the DEF position due to some pressure or temperature switch anomaly on a cold start/cold engine.
During a cold spell and upon initial start with the hood up you might consider eyeballing the compressor to make sure that the compressor clutch is engaged; in the DEF position of course.
My first guess would be a leaky heater core, but if it was that and you felt the windshield when this happened, it wouldn’t feel like water, more oily-like than water. Harder to wipe clean. I wonder, could it be maybe the AC is supposed to remove water vapor, but somehow that water vapor remains inside because of a stuck drain somewhere? I’ve never had car AC, but isn’t there something called the evaporator drain that clogs up sometimes? Could that be it?